1. Field of the Invention
The present invention relates to a resonator using a three-dimensional photonic crystal, more particularly, though not exclusively, the present invention relates to a resonator formed from a defect in a photonic crystal.
2. Description of the Related Art
Yablonovitch has proposed an idea of controlling the transmission and reflection characteristics of electromagnetic waves by structures smaller than or equal to a wavelength in Physical Review Letters, Vol. 58, p. 2059, 1987. This document has taught that the transmission and reflection characteristics of electromagnetic waves can be controlled by periodically arranging structures smaller than or equal to a wavelength. A photonic crystal can be such a unit of periodically arranged structures, and suggests that a reflection mirror exhibiting no optical loss, namely, a reflectance of about 100%, can be achieved in a specific wavelength region. This region is referred to as a photonic band gap, likened to an energy gap of a semiconductor. In particular, a three-dimensional microscopic periodic structure can produce a photonic band gap for all the light entering from any direction (hereinafter referred to as complete photonic band gap). The complete photonic band gap can have various applications (e.g., light emitting devices, control of spontaneous emission).
Woodpile structures in which columnar structures are periodically stacked in a layered manner have been discussed in, for example, U.S. Pat. No. 5,335,240 and Japanese Patent Laid-Open No. 2004-6567. Japanese Patent Laid-Open No. 2004-6567 has also discussed a resonator using a woodpile structure having a defect inside. The defect is formed in a rectangular solid shape with the same thickness as that of the columnar structure, and is present among the columnar structures. In general, a resonator produced by a defect formed in a photonic crystal has plural resonance modes. Resonators used in light emitting devices or wavelength selection filters have designed confinement effects and satisfy resonance requirements for a designed resonant wavelength. In addition, in order to avoid the influence of other resonance modes having a wavelength close to the designed wavelength, for example, to reduce the effect of mode hopping in a laser, one can provide a large difference between the designed resonant wavelength and the wavelengths of the resonant in the other resonance modes.
The periodicity around the defect of the periodic structure can be increased to increase the reflectance. Japanese Patent Laid-Open No. 2004-6567 has also discussed another approach in which the difference in resonant wavelength between adjacent resonance modes is controlled by varying the lengths of two sides other than the thickness of rectangular solid defect, and by shifting the position of the defect with respect to the columnar structures. Unfortunately, the shift of the position of the defect makes the periodic structure asymmetrical, so that the energy distribution of the electromagnetic field in the resonator is also made asymmetrical. If the resonator is used in a laser, such asymmetry causes a large deflection of the orientation of the emitted light and may result in a critical problem. In addition, the maximum of the shift in the position of the defect is ¼ of the period of columnar structure arrangement, making the choices of designed resonant wavelength limited.